There has been a gradual shift in focus from circuit-switched, speech centric usage, towards packet-switched data traffic in the current mobile cellular standards and technologies like GSM (Global System for Mobile Communications) and WCDMA (Wideband Code-Division Multiple Access). Along with that, a strive for higher data rates have resulted in additions to the standards and technologies, like EDGE (Enhanced Data rates for Global Evolution) for GSM and HSDPA (High-Speed Downlink Packet Access) for WCDMA. The complexity of radio transceiver circuitry increases with increasing data rates. For example, the requirements on signal purity in transmitting and receiving circuitry increases in order to meet the requested high data rates.
A mechanism that makes reception at high data rates more difficult than reception at low data rates is that the effective energy per transmitted information bit is reduced with increasing data rate. This is a reason for the increased requirements on signal purity in the transmitting and receiving circuitry. A traditional solution to improve the signal purity when the energy per bit decreases is to design a radio transceiver with significantly harder requirements with respect to e.g. noise figure, frequency and timing drift, phase noise, image rejection, and introduction of DC offsets to the received signal. Hence, the complexity of a radio transceiver tends to increase as data rates increase. A problem with this type of traditional solution is that the resulting circuitry will occupy a larger chip area, which results in higher cost. Furthermore, the power consumption of such circuitry is relatively high, leading to reduced battery operating time. These negative side effects are constantly present, despite the fact that the highest achievable data rates are normally used relatively rarely. A reason for this is that a typical user utilizes the high data rates only occasionally. Another reason is that the highest data rates require very good radio conditions, which can typically be found within line of sight of a base station.